Observations of the isotropic diffuse gamma-ray emission with the Fermi Large Area Telescope Markus Ackermann SLAC National Accelerator Laboratory on behalf of the Fermi LAT collaboration Fermi Symposium, Nov. 2009, Washington DC

Fermi Symposium, 11/02/09-11/05/09 ● Markus Ackermann for the LAT collaboration ● 2 Main contributions to the Fermi gamma-ray sky EGRET EGB LAT (E>100 MeV) 9 month observation Inverse Compton  0 -decay Bremsstrahlung Galactic diffuse emission (CR interactions with the interstellar medium) Resolved sources Isotropic diffuse emission Residual cosmic rays surviving background rejection filters misreconstructed -rays from the earth albedo

Fermi Symposium, 11/02/09-11/05/09 ● Markus Ackermann for the LAT collaboration ● 3 Potential contributions to the isotropic diffuse continuum gamma-ray emission in the LAT energy range (100 MeV-300 GeV):  unresolved point sources Active galactic nuclei (see talk by M. Ajello) Star-forming galaxies Gamma-ray bursts  diffuse emission processes UHE cosmic-ray interactions with the Extragalactic Background Light Structure formation large Galactic electron halo WIMP annihilation The isotropic diffuse gamma-ray emission  Isotropic diffuse flux contribution from unresolved sources depends on LAT point source sensitivity  Contribution expected to decrease with LAT observation time Incomplete collection of model predictions (Dermer, 2007)

Fermi Symposium, 11/02/09-11/05/09 ● Markus Ackermann for the LAT collaboration ● 4 Cosmic-ray background  Primary cosmic-rays + secondary CR produced in earth atmosphere  Charged and neutral cosmic-rays outnumber celestial gamma-rays by many orders of magnitude  CR contamination strongly suppressed by Anti-coincidence detector (ACD) veto and multivariate analysis of event properties  Residual CR produce unstructured, quasi-isotropic background (after sufficient observation time) primary protons alpha + heavy ion EGRET EGB sec. protons sec. positrons sec. electrons albedo-gammas prim. electrons

Fermi Symposium, 11/02/09-11/05/09 ● Markus Ackermann for the LAT collaboration ● 5 Data selection for the analysis of the isotropic flux  3 event classes defined in standard LAT event selection  LAT isotropic flux expected to be below EGRET level (factor »10 improvement in point source sensitivity)  LAT on-orbit background higher than predicted from pre-launch model  More stringent background rejection developed for this analysis  Event parameters used: Shower shape in Calorimeter Charge deposit in Silicon tracker Gamma-ray probability from classification analysis Distance of particle track from LAT corners  LAT standard event classes: Event classBackground contamination transient<~ 100 x EGRET EGB flux source<~ 20 x EGRET EGB flux diffuse<~ 1 x EGRET EGB flux MC study (Atwood et al. 2009)

Fermi Symposium, 11/02/09-11/05/09 ● Markus Ackermann for the LAT collaboration ● 6 Performance of the dedicated event selection  Improved residual background suppression compared to diffuse class  Improved agreement between simulation and data from rejection of hadronic shower and heavy ions Uncertainty: +50%/-30%  Retained effective area for  -rays simulation

Fermi Symposium, 11/02/09-11/05/09 ● Markus Ackermann for the LAT collaboration ● 7 Analysis technique  Pixel-by-pixel max. likelihood fit of |b|>10º sky equal-area pixels with ~ 0.8 deg 2 (HEALPIX grid) sky-model compared to LAT data point source /diffuse intensities fitted simultaneously 9 independent energy bins, 200 MeV GeV 10 month of LAT data, 19 Ms observation time  Sky model: Maps of Galactic foreground  -rays considering individually contributions from IC and local HI Individual spectra of TS>200 (~>14  ) point sources from LAT catalog Map of weak sources from LAT catalog Solar IC and Disk emission Spectrum of isotropic component  Subtraction of residual background (derived from Monte Carlo simulation) from isotropic component + + = LAT sky gal. diffuse point sources isotropic

Fermi Symposium, 11/02/09-11/05/09 ● Markus Ackermann for the LAT collaboration ● 8 Model of the Galactic foreground  Diffuse gamma-ray emission of Galaxy modeled using GALPROP  Spectra of dominant high-latitude components fit to LAT data: Inverse Compton emission (isotropic ISRF approximation) Bremsstrahlung and  0 -decay from CR interactions with local (7.5kpc < r < 9.5kpc) atomic hydrogen (HI)  HI column density estimated from 21-cm observations and E(B-V) magnitudes of reddening  4 kpc electron halo size for Inverse Compton component (2kpc - 10kpc tested) -ray emission model HI (7.5kpc < r < 9.5kpc) -ray emission model Inverse Compton scattering

Fermi Symposium, 11/02/09-11/05/09 ● Markus Ackermann for the LAT collaboration ● 9 The LAT isotropic diffuse flux (200 MeV – 100 GeV) |b| > 10º extragalactic diffuse CR background LAT  Spectrum can be fitted by power law:  = /  Flux above 100 MeV: F 100 = / x  cm- 2 s- 1 sr -1 (extrapolated)  Foreground modeling uncertainty not included in error bands PRELIMINARY

Fermi Symposium, 11/02/09-11/05/09 ● Markus Ackermann for the LAT collaboration ● 10 Systematic uncertainties from foreground modeling  RMS of residual map (averaged over 13.4 deg 2 bins) is 8.2%, 3.3 % expected from statistics  Residuals show some correlation to structures seen in the galactic foreground emission  Foreground model is not perfect.  Impact of foreground model variations on derived EGB intensity studied:  Table items are NOT independent and cannot be added to provide overall modeling uncertainty Flux in band200 MeV – 400 MeV1.6 GeV GeV51 GeV – 102 GeV Extragalactic2.4 +/ / /- 2.9 HI column density+0.1 / / / -1.1 Halo size + IC+0.1 / / / -0.5 CR propagation model+0.1 / / / -0.1 Subregions of |b|> / / / -0.9 x cm -2 s -1 sr -1 x cm -2 s -1 sr -1 x cm -2 s -1 sr -1

Fermi Symposium, 11/02/09-11/05/09 ● Markus Ackermann for the LAT collaboration ● 11 Comparison with EGRET results  Considerably steeper than the EGRET spectrum by Sreekumar et al.  No spectral features around a few GeV seen in re-analysis by Strong et al Flux, E>100 MeVspectral index LAT (this analysis)1.03 +/ / EGRET (Sreekumar et al., 1998)1.45 +/ / EGRET (Strong et al. 2004)1.11 +/ LAT + resolved sources below EGRET sensitivity1.19 +/ / x cm -2 s -1 sr -1 PRELIMINARY

Fermi Symposium, 11/02/09-11/05/09 ● Markus Ackermann for the LAT collaboration ● 12 Summary  A new low-background data selection was developed to obtain a measurement of the EGB. This data selection will be made public with the next update of the Fermi event classification.  The EGB found by the LAT is compatible with a simple power law of index 2.41+/-0.05 between 200 MeV and 100 GeV.  It is softer than the EGRET spectrum and does not show distinctive peaks (compared at EGRET sensitivity level).  ~ 15% of the EGRET EGB is resolved into sources by the LAT.  From Blazar population study: ~20%-30% of LAT EGB is due to unresolved Blazars (see M. Ajello’s talk).  Ongoing work to extend the energy range and reduce systematic uncertainties of this measurement.

Fermi Symposium, 11/02/09-11/05/09 ● Markus Ackermann for the LAT collaboration ● 13

Fermi Symposium, 11/02/09-11/05/09 ● Markus Ackermann for the LAT collaboration ● 14 Cosmic Ray background in data and simulation  Sample A: events classified as  -rays by on-board filters, |b|>45 deg  Sample B: events accepted in medium purity (“source”), but rejected in high purity (“diffuse”) standard event class, |b|>45 deg Both samples are strongly dominated by CR background ! Sample A  bulk of the CR background Sample B  extreme tails of CR distribution which mimic -rays + shower shape and charge deposit cuts Tails of the CR distribution agree within +50%/- 30%  uncertainty of the CR background for this analysis

Fermi Symposium, 11/02/09-11/05/09 ● Markus Ackermann for the LAT collaboration ● 15 Data selection for the analysis of the isotropic diffuse background  Example for improved background rejection: Transverse shower size in Calorimeter clean dataset (observations with high  -ray flux, low CR flux) contaminated dataset (observations with low  -ray flux, high CR flux) predicted distribution from LAT simulation clean contaminated simulation

Fermi Symposium, 11/02/09-11/05/09 ● Markus Ackermann for the LAT collaboration ● 16 The Fermi Large Area Telescope  Energy range: 100 MeV – 300 GeV  Peak effective area: > 8000 cm 2 (standard event selection)  Field of view:2.4 sr  Point source sensitivity (>100 MeV) : 3x10 -9 cm -2 s -1  No consumables onboard LAT  Steady response over time expected  Standard operation in ‘sky survey’ mode allows almost flat exposure of the sky LAT effective area for vertically incident -rays LAT 3GeV (1-year sim.) cm 2 s cm 2 s